Recently, there are major developments in telephone communications. One example is the tremendous growth in digital cellular telephones. Another example is the use of packet-based data networks, such as those conforming to the Internet Protocol (IP), to carry voice communications (as opposed to using circuit-switched networks). This new technology is called voice-over IP (VoIP). In these applications, voice signals are digitized and travel on digital communication channels. The digital data is processed using digital signal processors (DSPs) and/or field programmable gate arrays (FPGAs). One important task performed by the DSP/FPGA is the elimination of echo.
In telephony applications, “echo” is defined as the reflection of the caller's voice back to the caller through the phone lines. Echo cancellation is the elimination of echo in telephone communications. There are several causes of echo. One cause, called “line” echo, is created when an electrical signal encounters an impedance mismatch at one end of the line, such as that caused by a 2 to 4 wire hybrid in an analog phone system. The echo is exacerbated by distance and by certain kinds of network equipment. Echo delayed by 30 ms or more is generally noticeable to the user, and delays greater than 50 ms affect the quality of the conversation.